The invention relates to the control of the vertical deflection of a spot scanning a screen, for example, screens of television sets and/or computer monitors.
A spot scans a screen in two orthogonal directions, namely a line scan and a vertical scan. The line scan takes place at a much higher frequency than the vertical scan. For example, for a television, the line scan may take place at 15625 Hz, whereas the vertical scan takes place, for example, at 50 Hz. When the spot has undergone a forward line scan, a line retrace then occurs in order to scan the next line. Simultaneously with the line scan, the vertical scan allows the spot to scan the screen from the top down. At the end of the downward vertical scan, a rapid retrace of the spot to the top and to the left of the screen (frame retrace) then occurs, this being commonly called xe2x80x9cFlybackxe2x80x9d by those skilled in the art.
The vertical deflection of the spot therefore comprises a downward vertical scan phase followed by a spot flyback phase. The vertical deflection is generated by a vertical deflection circuit, which is conventionally an inductive circuit generating a magnetic field for deflecting the spot. The conventional devices for controlling vertical deflection circuits (or more simply xe2x80x9cvertical deflectorsxe2x80x9d) are formed by linear class AB amplifiers, i.e. an amplifier with the output stage formed by transistors working in their linear range and consequently producing a continuous output voltage. The amplifier amplifies a reference input signal, generally a sawtooth signal, so as to deliver a signal for controlling the vertical scan of the spot. Moreover, in order to ensure flyback of the spot, these control devices are equipped to generate an overvoltage at the terminals of the deflector.
Currently, such control devices have an actual efficiency of about 50% and generally require relatively large heat sinks.
An object of the invention is to remedy this drawback by increasing the efficiency of such a control circuit, while at the same time reducing the complexity of the heat sinks, so as to reduce the overall cost of the control device.
Yet another object of the invention is to produce such control devices in the form of integrated circuits.
The invention therefore provides a method for controlling a circuit for the vertical deflection of a spot scanning a screen. In the method, the vertical scan of the spot is controlled by an output amplifier stage supplied by a main supply (for example xc2x115 volts) and the flyback of the spot is initiated by delivering an overvoltage, produced by an auxiliary supply, to the vertical deflection circuit.
The control device includes an output amplifier stage having at least two transistors which are made to operate in Class-D operation (i.e. it is alternately turned on and off) at least for controlling the vertical scan of the spot. The overvoltage is delivered by controlling a first two-way switch connected to the output stage and to the auxiliary supply so as to allow a current to flow through this first switch. In addition, a second two-way switch connected between a first transistor of the output stage and a first terminal (for example +Vcc) of the main supply is controlled so as to prevent a current from flowing through the first switch during the vertical scan of the spot and thus to prevent the overvoltage being delivered during the vertical scan of the spot.
In Class-D operation, the transistors operate in on/off mode, i.e. when one of them is off the other is on, and vice versa. In Class-D operation, when a transistor is off it does not dissipate power, while when it is on it dissipates very little power. The output voltage of the output stage is a xe2x80x9chighxe2x80x9d or xe2x80x9clowxe2x80x9d signal. If the duration of the high state is equal to that of the low state, the average value of the output signal is zero. By modulating the duty cycle of the control signal (control pulses of variable width) of the filters and by filtering the high-frequency component, an average voltage is recovered which varies over time. This variation is very slow compared with the chopping frequency (the frequency of the control signal). It is thus possible to achieve an efficiency that may be as high as 90%, as opposed to approximately 45% to 50% in the case of Class-AB operation.
Furthermore, in combination with this Class-D operation, the invention provides for the delivery of the overvoltage by controlling a first two-way switch so as to allow a current to flow through this first switch. Moreover, it is necessary to provide the second two-way switch which is controlled so as to prevent a current from flowing through the first switch during the vertical scan of the spot, and in particular when, in a second phase of the forward scan, the second transistor of the output stage is off.
All the transistors, whether those of the output stage or else those advantageously forming the various two-way switches, may be bipolar transistors. This being the case, especially for the purpose of having an integrated-circuit construction, it is particularly advantageous for these transistors to be isolated-gate field-effect transistors (MOS transistors), for example of the n-channel type.
Although in theory it is not necessary to connect a diode to each transistor in antiparallel to its terminals, this proves to be preferable, especially if bipolar transistors are used, so as to allow good conduction of the transistors in both directions, but also when MOS transistors are used so as to avoid an increase in the threshold voltage of these transistors, which generally operate with large signals. In practice, the antiparallel-connected diode is produced by connecting the source of the transistor to the substrate (or bulk). More generally, it is pointed out here that, in the context of the present invention, a diode is said to be connected in antiparallel with the terminals of a transistor when the anode of the diode is connected to the source of the transistor while the cathode of the diode is connected to the drain of the transistor (when this a MOS transistor).
According to one embodiment of the invention, the first two-way switch comprises a transistor and a diode connected in antiparallel with the terminals of the transistor, the anode of the diode being connected to the first transistor of the output stage. The second two-way switch, which makes it possible to avoid delivery of the overvoltage during control of the vertical scan of the spot, comprises a transistor connected in series between the first transistor of the output stage and the first terminal of the main supply (for example +Vcc). This second two-way switch also comprises a diode connected in antiparallel with the terminals of the transistor, the cathode of this diode being connected to the output stage and the anode being connected to the said first terminal of the main supply.
The vertical scan of the spot comprises a first phase during which the current flows into the vertical deflection circuit and a second phase during which the current flows from the vertical deflection circuit. Moreover, the flyback of the spot comprises a first phase during which the current flows from the vertical deflection circuit and a second phase during which the current flows into the vertical deflection circuit. The transistor of the first two-way switch (i.e. the one which allows the overvoltage to be delivered) is turned off during the vertical scan, while the second two-way switch (i.e. the one which prevents the overvoltage being delivered during the vertical scan) is controlled so as to turn the transistor of this second two-way switch on at least when the second transistor of the output stage is off during the second phase of the vertical scan. Furthermore, the transistor of the second two-way switch is off during the flyback of the spot, while the transistor of the first two-way switch is turned on at least during the second phase of the flyback of the spot.
In practice, for reasons of simplicity of control and of power dissipation, it is preferable to turn the transistor of the second two-way switch on right from the start of the vertical scan of the spot and, during the flyback of the spot, to turn the transistor of the first two-way switch on as soon as the transistor of the second two-way switch has been turned off.
Variations are possible for generating the overvoltage necessary for flyback of the spot. An external auxiliary supply or else a so-called internal auxiliary supply, generally including a capacitor, called an auxiliary or flyback capacitor, may be provided. In a first variant (external auxiliary supply), the first two-way switch is connected in series between the auxiliary supply and the first transistor of the output stage. In a second variant (internal auxiliary supply), the auxiliary supply comprises an auxiliary capacitor, a first terminal of which is connected, on the one hand, to the first terminal of the main supply (for example +Vcc) via the first two-way switch and, on the other hand, to a second terminal of the main supply (xe2x88x92Vcc for example, or else earth) via a controllable auxiliary switch. The second terminal of the capacitor (for example, the positive terminal) is connected to the first transistor of the output stage and to the second two-way switch. The auxiliary switch is then open when the transistor of the first two-way switch is on, and vice versa.
More specifically, according to one embodiment, the auxiliary switch is closed during the vertical scan of the spot and this auxiliary switch is open during the flyback of the spot. In the variant using the internal auxiliary supply, the transistor of the first two-way switch is connected between the first terminal of the auxiliary capacitor and the first terminal (for example +Vcc) of the main supply. It is therefore particularly advantageous to use the energy stored in the auxiliary capacitor to turn on the transistor of this first two-way switch.
Thus, according to one embodiment of the invention, the gate of the transistor of this first two-way switch is connected to the first terminal of the auxiliary capacitor in order to turn this transistor on and the gate of the transistor of the first two-way switch is connected to its source in order to turn this transistor off.
In general, a bootstrap capacitor is advantageously connected between the source and the gate of the first transistor of the output stage so as to make it easier to turn it on. In one embodiment of the invention, the energy stored in this bootstrap capacitor is advantageously used to turn on the transistor of the second two-way switch during the vertical scan of the spot. Furthermore, this embodiment, using the bootstrap capacitor, is compatible with both of the variants relating to the auxiliary supply (external auxiliary supply or internal auxiliary supply via the auxiliary capacitor).
More specifically, the bootstrap capacitor is advantageously connected to the gate of the transistor of the second two-way switch during the vertical scan of the spot and the gate of the transistor of this second two-way switch is connected to its source during the flyback of the spot.
According to one embodiment of the invention, allowing the transistor to be controlled more simply, the transistors of the Class-D output stage are controlled on the basis of a reference signal during the vertical scan phase of the spot and the first transistor of the output stage is turned on during the flyback phase of the spot. Moreover, as a variation, it is possible to amplify the reference signal feedback-modified by the current passing through the deflection circuit so as to deliver an error signal. The point when the error signal crosses a predetermined threshold is then detected and the flyback phase of the spot is triggered when the error signal crosses the said threshold.
The invention is also directed to a device for controlling a circuit for the vertical deflection of a spot scanning a screen, this device comprising an output amplifier stage supplied by a main supply for controlling the vertical scan of the spot, and an auxiliary supply capable of delivering an overvoltage to the vertical deflection circuit for flyback of the spot. According to a general characteristic of the invention, the output stage is a stage comprising at least two transistors capable of operating in Class-D operation, at least for controlling the vertical scan of the spot, this stage being associated with a smoothing filter connected to the common terminal of the two transistors. Moreover, the device includes a first controllable two-way switch connected to the output stage and to the auxiliary supply, as well as a second controllable two-way switch connected between a first transistor of the output stage and a first terminal (+Vcc for example) of the main supply.
The device also includes a controller for controlling: the transistors of the output stage in Class-D operation; the first switch so as to allow a current to flow through this first switch and to allow the overvoltage to be delivered; and the second switch so as to prevent a current from flowing through the first switch during the vertical scan of the spot and thus prevent the overvoltage from being delivered during the vertical scan of the spot.
According to one embodiment of the invention, during control of the vertical scan of the spot, the controller controls the transistors of the Class-D output stage on the basis of a reference signal present as an input to the device, opens the first two-way switch and closes the second two-way switch. During control of the flyback of the spot, the controller turns the first transistor of the output stage on, opens the second two-way switch and closes the first switch.
According to one embodiment of the invention, the device includes a preamplification stage which receives the reference input signal feedback-modified by the current passing through the vertical deflection circuit. This preamplification stage delivers an error signal. The device further includes a detector for detecting when the error signal crosses a predetermined threshold. The controller then triggers the flyback phase of the spot when the error signal crosses the threshold.
It is also possible, as a variant, for the output stage to be formed by four transistors in a complete bridge arrangement. In other words, the output stage then comprises two other transistors capable of operating in the switching mode in an alternating fashion at least for controlling the vertical scan of the spot. The smoothing filter is then connected between the common terminal of the first two transistors and the common terminal of the other two transistors of the output stage. The smoothing filter is further distributed on either side of the vertical deflection circuit. Of course, in this case, the overvoltage generator necessary for flyback of the spot, as well as the overvoltage inhibitor for inhibiting the delivery during the vertical scan of the spot, are connected only to a single pair of transistors of the bridge.
The invention is also directed to a television set or a computer monitor comprising a screen, a circuit for the vertical deflection of a spot scanning the screen and a device for controlling the vertical deflection circuit, as described above.